|Número de publicación||US7155047 B2|
|Tipo de publicación||Concesión|
|Número de solicitud||US 10/324,991|
|Fecha de publicación||26 Dic 2006|
|Fecha de presentación||20 Dic 2002|
|Fecha de prioridad||20 Dic 2002|
|También publicado como||US20040120565|
|Número de publicación||10324991, 324991, US 7155047 B2, US 7155047B2, US-B2-7155047, US7155047 B2, US7155047B2|
|Inventores||Scott David Wollenweber|
|Cesionario original||General Electric Company|
|Exportar cita||BiBTeX, EndNote, RefMan|
|Citas de patentes (8), Otras citas (1), Citada por (5), Clasificaciones (9), Eventos legales (3)|
|Enlaces externos: USPTO, Cesión de USPTO, Espacenet|
This invention relates generally to imaging systems, and more particularly, to methods and apparatus for retrospectively assessing image quality of images from imaging systems.
At least one known PET transmission scan is acquired using a rotating source of dual-511 keV gamma rays, such as the Ge-68 rod sources implemented on imaging systems, such as a PET Advance system from General Electric Medical Systems, Waukesha, Wis. During a multiple axial field-of-view (AFOV) study, image quality loss at the end slices of each axial FOV due to lower system sensitivity on these slices may cause an operator to question the overall image quality of the study. This loss of sensitivity is typically corrected by overlapping the axial fields-of-view (AFOVs) by at least one slice of data between two adjacent AFOVs. However, there is often still some degradation in the statistical quality of the overlap slices due to larger patient size (larger patient size typically equals a lower statistical quality), depleting transmission rod source strength, or a shorter transmission scan duration. Often, the attenuation-corrected emission images are viewed in an orthogonal reformat, and a coronal view may show these overlap areas as ‘bands’ of different image quality from surrounding areas. These bands may cause an image reader to question the quality of other areas of the image set. There are several known methods to correct this problem, such as lengthening the transmission scan duration or replacing one or both of the radioactive rod sources. However, often in the clinical setting one can not predict the necessity of pin replacement or probability of occurrence of image quality problems prior to performing the patient scan.
In one aspect, a method for retrospectively measuring a plurality of transmission datasets collected using an imaging system is provided. The method includes acquiring a plurality of multi-axial field-of-view (AFOV) datasets, the datasets including a plurality of pairs of adjacent images, determining a correlation value for each pair of adjacent images, calculating a derivative for the correlation values, and generating an indication when the derivative exceeds a predetermined threshold.
In another aspect, a method for retrospectively measuring a plurality of transmission datasets collected using Positron Emission Tomography (PET) system is provided. The method includes acquiring a plurality of multi-axial field-of-view (AFOV) datasets, the datasets including a plurality of pairs of adjacent images, determining a correlation value for each pair of adjacent images in accordance with
calculating a first derivative and a second derivative for the correlation values using a three-dimensional Lagrangian interpolation algorithm, and generating an indication when the second derivative exceeds a predetermined threshold.
In a further aspect, an imaging system including a radiation source, a radiation detector, and a computer operationally coupled to the radiation source and the radiation detector is provided. The computer is configured to acquire a plurality of multi-axial field-of-view (AFOV) datasets, the datasets including a plurality of pairs of adjacent images, determine a correlation value for each pair of adjacent images, calculate a derivative for the correlation values, and generate an indication when the derivative exceeds a predetermined threshold.
In still another aspect, a Positron Emission Tomography (PET) system including a radiation source, a radiation detector, and a computer operationally coupled to the radiation source and the radiation detector is provided. The computer is configured to acquire a plurality of multi-axial field-of-view (AFOV) datasets, the datasets including a plurality of pairs of adjacent images, determine a correlation value for each pair of adjacent images in accordance with
calculate a first derivative and a second derivative for the correlation values using a three-dimensional Lagrangian interpolation algorithm, and generate an indication when the second derivative exceeds a predetermined threshold.
In still another further aspect, a computer readable medium encoded with a program is provided. The medium is configured to instruct a computer to acquire a plurality of multi-axial field-of-view (AFOV) datasets, the datasets including a plurality of pairs of adjacent images, determine a correlation value for each pair of adjacent images, calculate a derivative for the correlation values, and generate an indication when the derivative exceeds a predetermined threshold.
In some known CT imaging system configurations, an X-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an “imaging plane”. The X-ray beam passes through an object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated radiation beam received at the detector array is dependent upon the attenuation of an X-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam intensity at the detector location. The intensity measurements from all the detectors are acquired separately to produce a transmission profile.
In third generation CT systems, the X-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged such that the angle at which the X-ray beam intersects the object constantly changes. A group of X-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the X-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display. Current PET scanners incorporate a process similar to that found in CT, in that a map or the object attenuation can be generated. A common method to perform this attenuation measurement includes use of rotation rod sources containing positron-emitting radionuclides. The rods rotate outside the patient bore, but inside the diameter of the PET detector ring. Annihilation events occurring in the rods can send one photon into a near-side detector while the pair photon traverses the object of interest in a manner similar to the CT X-ray. The data found from this method contains essentially the same information as that found from the CT method except for the statistical quality of the resultant data. In the rotating rod case, the statistical quality is orders of magnitude inferior to most common CT scans. For the PET purpose, data acquired in this manner is used to correct for the attenuation seen in the object by the 511 keV photons, which is often the most substantial correction performed on the PET data.
To reduce the total scan time, a “helical” scan may be performed. To perform a “helical” scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
Reconstruction algorithms for helical scanning typically use helical weighing algorithms that weight the collected data as a function of view angle and detector channel index. Specifically, prior to a filtered backprojection process, the data is weighted according to a helical weighing factor, which is a function of both the gantry angle and detector angle. The weighted data is then processed to generate CT numbers and to construct an image that corresponds to a two dimensional slice taken through the object.
At least some CT systems are configured to also perform Positron Emission Tomography (PET) and are referred to as PET-CT systems. Positrons are positively charged electrons (anti-electrons) which are emitted by radio nuclides that have been prepared using a cyclotron or other device. The radionuclides most often employed in diagnostic imaging are fluorine-18 (18F), carbon-11 (11C), nitrogen-13 (13N), and oxygen-15 (15O). Radionuclides are employed as radioactive tracers called “radiopharmaceuticals” by incorporating them into substances such as glucose or carbon dioxide. One common use for radiopharmaceuticals is in the medical imaging field.
To use a radiopharmaceutical in imaging, the radiopharmaceutical is injected into a patient and accumulates in an organ, vessel or the like, which is to be imaged. It is known that specific radiopharmaceuticals become concentrated within certain organs or, in the case of a vessel, that specific radiopharmaceuticals will not be absorbed by a vessel wall. The process of concentrating often involves processes such as glucose metabolism, fatty acid metabolism and protein synthesis. Hereinafter, in the interest of simplifying this explanation, an organ to be imaged including a vessel will be referred to generally as an “organ of interest” and the invention will be described with respect to a hypothetical organ of interest.
After the radiopharmaceutical becomes concentrated within an organ of interest and while the radionuclides decay, the radionuclides emit positrons. The positrons travel a very short distance before they encounter an electron and, when the positron encounters an electron, the positron is annihilated and converted into two photons, or gamma rays. This annihilation event is characterized by two features which are pertinent to medical imaging and particularly to medical imaging using photon emission tomography (PET). First, each gamma ray has an energy of approximately 511 keV upon annihilation. Second, the two gamma rays are directed in nearly opposite directions.
In PET imaging, if the general locations of annihilations can be identified in three dimensions, a three dimensional image of radiopharmaceutical concentration in an organ of interest can be reconstructed for observation. To detect annihilation locations, a PET camera is employed. An exemplary PET camera includes a plurality of detectors and a processor which, among other things, includes coincidence detection circuitry.
The coincidence circuitry identifies essentially simultaneous pulse pairs which correspond to detectors which are essentially on opposite sides of the imaging area. Thus, a simultaneous pulse pair indicates that an annihilation has occurred on a straight line between an associated pair of detectors. Over an acquisition period of a few minutes millions of annihilations are recorded, each annihilation associated with a unique detector pair. After an acquisition period, recorded annihilation data can be used via any of several different well known image reconstruction methods to reconstruct the three dimensional image of the organ of interest.
As used herein, an element or step recited in the singular and preceded with the word “a” or “an” should be understood as not excluding plural the elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.
Also as used herein, the phrase “reconstructing an image” is not intended to exclude embodiments of the present invention in which data representing an image is generated but a viewable image is not. Therefore, as used herein the term “image” broadly refers to both viewable images and data representing a viewable image. However, many embodiments generate (or are configured to generate) at least one viewable image.
Rotation of gantry 12 and the operation of X-ray source 14 are governed by a control mechanism 26 of PET-CT system 10. Control mechanism 26 includes an X-ray controller 28 that provides power and timing signals to X-ray source 14 and a gantry motor controller 30 that controls the rotational speed and position of gantry 12. A data acquisition system (DAS) 32 in control mechanism 26 samples analog data from detector elements 20 and converts the data to digital signals for subsequent processing. An image reconstructor 34 receives sampled and digitized X-ray data from DAS 32 and performs high-speed image reconstruction. The reconstructed image is applied as an input to a computer 36 which stores the image in a storage device 38.
Computer 36 also receives commands and scanning parameters from an operator via console 40 that has a keyboard. An associated cathode ray tube display 42 allows the operator to observe the reconstructed image and other data from computer 36. The operator supplied commands and parameters are used by computer 36 to provide control signals and information to DAS 32, X-ray controller 28 and gantry motor controller 30. In addition, computer 36 operates a table motor controller 44 which controls a motorized table 46 to position patient 22 in gantry 12. Particularly, table 46 moves portions of patient 22 through gantry opening 48.
In one embodiment, computer 36 includes a device 50, for example, a floppy disk drive or CD-ROM drive, for reading instructions and/or data from a computer-readable medium 52, such as a floppy disk or CD-ROM. In another embodiment, computer 36 executes instructions stored in firmware (not shown). Computer 36 is programmed to perform functions described herein, and as used herein, the term computer is not limited to just those integrated circuits referred to in the art as computers, but broadly refers to computers, processors, microcontrollers, microcomputers, programmable logic controllers, application specific integrated circuits, and other programmable circuits, and these terms are used interchangeably herein. PET-CT system 10 also includes a plurality of PET detectors. The PET detectors and detector array 18 both detect radiation and are both referred to herein as radiation detectors. In one embodiment, PET-CT system 10 is a Discovery LS PET-CT system commercially available from General Electric Medical Systems, Waukesha Wis., and configured as herein described, including the ability to acquire the attenuation information from both the CT and the rotating rod source(s) incorporated into the PET gantry, as described previously. In another embodiment, system 10 performs at least one of a CT and PET imaging, but not both. In an alternative embodiment, imaging system 10 is an imaging modality other than CT and PET.
Although the specific embodiment mentioned above refers to a third generation CT system and a PET imaging system, the methods described herein equally apply to fourth generation CT systems (stationary detector—rotating X-ray source), fifth generation CT systems (stationary detector and X-ray source) or other PET-only or nuclear systems wherein a rod-source attenuation measurement system is incorporated.
Additionally, although the herein described methods are described in a medical setting, it is contemplated that the benefits of the invention accrue to non-medical imaging systems such as those systems typically employed in an industrial setting or a transportation setting, such as, for example, but not limited to, a baggage scanning system for an airport or other transportation center. The benefits also accrue to micro PET and CT systems which are sized to study lab animals as opposed to humans.
In the exemplary embodiment, a plurality of multi-axial field-of-view (AFOV) datasets (i.e. slices) are acquired using imaging system 10. A plurality of correlation values are generated in accordance with:
In use, the correlation value is parameterized by defining two ‘neighborhoods’ of images. For example, a plurality of temporally sequential datasets are produced as is known in the art. Adjacent datasets are then correlated to produce a plurality of correlation values. More specifically, a first dataset and a second subsequent dataset are used to generate the correlation value using Equation 1. A correlation value is then generated using the second dataset and a third subsequent dataset. In the exemplary embodiment, a correlation value is generated for all the acquired datasets in a pairwise manner between adjacent datasets. The correlation values are then plotted to generate a correlation function, as in graph 70 shown in
In one embodiment, if second derivative 82 exceeds predetermined threshold 84, a transaxial smoothing of the transmission data is increased from the default value of using an 8 mm Gaussian filter to using a 12 mm Gaussian filter. Using a 12 mm Gaussian filter facilitates reducing a plurality of horizontal banding artifacts in a coronal view.
In the exemplary embodiment, the methods described herein can be run immediately after transmission scan acquisitions and reconstructions or on a daily basis. In use, an operator can measure and track over time a quantity of occurrences where the derivative function exceeds a threshold. As the occurrence rate increases, the data can be assessed to determine a proper action to take, such as, but not limited to, lengthening a transmission scan time and replacing at least one radioactive rod source. The methods described herein can also be used after generation of at least one software executable program, in an untended manner with report out of relevant information, such as threshold used, number of frames of transmission data tested, start date for test, end date for test, and number of occurrences above threshold.
The methods described herein facilitate retrospective measurement of the quality of transmission scans acquired on multi-AFOV datasets. In use, by tracking this image quality, and its degradation over time marked by number of occurrences of problems, an operator can collect data which can then be used to predict when and how to address the problems associated with transmission scan quality.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
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|Clasificación de EE.UU.||382/131, 250/363.04, 382/278|
|Clasificación internacional||G06K9/64, G01T1/166, G06K9/00, G06T11/00|
|20 Dic 2002||AS||Assignment|
Owner name: GE MEDICAL SYSTEMS GLOBAL TECHNOLOGY COMPANY, LLC,
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:WOLLENWEBER, SCOTT DAVID;REEL/FRAME:013612/0884
Effective date: 20021219
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